1. Field of the Invention
The present invention relates to a tablet input device of the resistance film type.
2. Description of the Related Art
FIG. 1 illustrates a prior art tablet input device. In the figure, 11 designates a transparent input tablet consisting of two stacked rectangular resistance layers 11X and 11Y in each of which a transparent resistance material such as indium oxide is applied to form a transparent resistance film. One of the resistance layers, the layer 11X is used as an X-axis tablet, and X-axis electrodes a12 and a13 are respectively formed on the side of the X-axis tablet. The other of the resistance layers, the layer 11Y is used as a Y-axis tablet, and Y-axis electrodes all and a14 are respectively formed on the sides of the Y-axis tablet. Driving transistors Tr2a and Tr3a are respectively connected to the X-axis electrodes a12 and a13, and transistors Tr1a and Tr4a to the Y-axis electrodes all and a14.
In this tablet input device, the transistors Tr1a and Tr2a are kept in the ON state during an input waiting state, and when an any point of the tablet face is pressed the voltage level of the electrode a12 changes. The timing of this level change functions as a trigger to start an operation of detecting a position.
In the tablet input device having the above-described configuration, since the two transistors Tr1a and Tr2a are required to be kept in the driven state during the input waiting state, with the result that a relatively large power is consumed. In the even that such a tablet input device is applied to a small electronic apparatus, its current consumption considerably affects the life of an battery used in the apparatus. In order reduce the power consumption, FETs (field effect transistors) may be used as the switching elements. However, FETs have generally a high driving voltage, and therefore it is difficult to design a practical circuit.